Isolation of a GCC repeat showing expansion in FRAXF, a fragile site distal to FRAXA and FRAXE.

Three folate-sensitive fragile sites, termed FRAXA, FRAXE and FRAXF, have been identified on the distal end of chromosome Xq. The first two contain expanded, hypermethylated and unstable CGG (or GCC) repeats within CpG islands. We now report the isolation of similar sequences responsible for the third fragile site, FRAXF. A 5-kilobase EcoRI fragment derived from a cosmid coincident with the cytogenetic anomaly detects expanded, methylated and unstable sequences in five individuals who exhibit fragile sites in distal Xq; these individuals have normal repeat lengths at both FRAXA and FRAXE. By sequence analysis, the expanded region contains a GCC repeat. PCR and sequence analysis of chromosomes from the general population indicates that the repeat is polymorphic (6 to 29 triplets), and is stable upon transmission.

A point mutation in the FMR-1 gene associated with fragile X mental retardation.

The vast majority of patients with fragile X syndrome show a folate-sensitive fragile site at Xq27.3 (FRAXA) at the cytogenetic level, and both amplification of the (CGG)n repeat and hypermethylation of the CpG island in the 5' fragile X gene (FMR-1) at the molecular level. We have studied the FMR-1 gene of a patient with the fragile X phenotype but without cytogenetic expression of FRAXA, a (CGG)n repeat of normal length and an unmethylated CpG island. We find a single point mutation in FMR-1 resulting in an lle367Asn substitution. This de novo mutation is absent in the patient's family and in 130 control X chromosomes, suggesting that the mutation causes the clinical abnormalities. Our results suggest that mutations in FMR-1 are directly responsible for fragile X syndrome, irrespective of possible secondary effects caused by FRAXA.

The full mutation in the FMR-1 gene of male fragile X patients is absent in their sperm.

Fragile X syndrome is characterized at the molecular level by amplification of a (CGG)n repeat and hypermethylation of a CpG island preceeding the open reading frame of the fragile X gene (FMR-1) located in Xq27.3. Anticipation in this syndrome is associated with progressive amplification of the (CGG)n repeat from a premutation to a full mutation through consecutive generations. Remarkably, expansion of the premutation to the full mutation is strictly maternal. To clarify this parental influence we studied FMR-1 in sperm of four male fragile X patients. This showed that only the premutation was present in their sperm, although they had a full mutation in peripheral lymphocytes. This might suggest that expansion of the premutation to the full mutation in FMR-1 does not occur in meiosis but in a postzygotic stage.

Characterization of the imprinted IPW gene: allelic expression in normal and tumorigenic human tissues.

IPW (Imprinted gene in the Prader-Willi syndrome region) is a recently identified paternally expressed gene. Previous work has demonstrated IPW expression in the human fetus and adult, with monoallelic expression in adult lymphoblasts and fibroblasts, and in fetal tissues. To further examine the expression of IPW, a series of experiments were carried out using RT-PCR to measure IPW expression in placentae and various fetal and tumor tissues. Biallelic expression of IPW was found in testicular germ cell tumor and bladder cancer cells, suggesting loss of imprinting in the latter case. Both H19 and Insulin-like growth Factor 2 (IGF2), two additional imprinted genes, also showed biallelic expression in those same tumors that demonstrated IPW biallelic expression. Of note, the naturally occurring parthenogenetic-derived mature teratoma unexpectedly expressed large amounts of IPW. Lastly, the pluripotent embryonal cancer cell line Tera-2 expressed IPW at the same level before and after differentiation induced by retinoic acid, suggesting that this gene functions in a 'housekeeping' capacity throughout cell growth. This was in contradistinction to H19 and IGF2, both of which showed significant transcriptional upregulation after Tera-2 cell differentiation.

Unique expression patterns of H19 in human testicular cancers of different etiology.

The expression pattern of the imprinted human H19 gene was investigated in testicular cancers of different etiology, as well as in normal testicular parenchyma, parenchyma without germ cells, and adjacent to testicular germ cell tumors of adolescents and adults (TGCTs), using RNase protection analysis, mRNA in situ hybridization and reverse-transcription polymerase chain reaction. While different total expression levels were detected in spermatocytic seminomas, lymphomas, a Sertoli cell tumor and Leydig cell tumors, none showed a disturbance of monoallelic expression. Strikingly, the majority of invasive TGCTs revealed expression of both parental alleles. The total level of expression highly correlated with differentiation lineage and stage of maturation, similar to that as reported during early normal embryogenesis. Biallelic expression could also be determined specifically in testis parenchyma containing the preinvasive lesion of this cancer. We therefore conclude that within the adult testis, biallelic H19 expression is specific for TGCTs, and that the level of expression is dependent on differentiation lineage and maturation stage. This is in agreement with the proposed primordial germ cell-origin of this cancer, and might be related to retention of embryonic characteristics in TGCTs. In addition, our data argue against H19 being a tumor suppressor gene.

Identification of the critical region of 12p over-representation in testicular germ cell tumors of adolescents and adults.

Cytogenetically, testicular germ cell tumors of adolescents and adults (TGCTs) are characterized by gain of 12p-sequences, most often through isochromosome formation (i(12p)). Fluorescence in situ hybridization (FISH) has shown that i(12p))-negative TGCTs also cryptically contain extra 12p-sequences. The consistency of 12p-over-representation in all histological subtypes of TGCTs, including their preinvasive stage, suggests that gain of one or more genes on 12p is crucial in the development of this cancer. So far, studies aimed at the identification of the relevant gene(s) were based on the 'candidate-gene approach'. No convincing evidence in favor of or against a particular gene has been reported. We combined conventional karyotyping, comparative genomic hybridization, and FISH to identify TGCTs with amplifications of restricted regions of 12p. Out of 49 primary TGCTs (23 without i(12p), 13 with and 13 unknown), eight tumors (six without i(12p) and two unknown) showed amplifications corresponding to 12p11.1-p12.1. Using bicolour-FISH, physical mapping, and semi-quantitative polymerase chain reactions, the size of the shortest region of overlap of amplification (SROA) was estimated to be between 1750-3000 kb. In addition, we mapped a number of genes in and around this region. While fourteen known genes could be excluded as candidates based on their location outside this region, we demonstrate that KRAS2, JAW1 and SOX5 genes are localized within the SROA. While KRAS2 and JAW1 map to the proximal border of the SROA, SOX5 maps centrally in the SROA. KRAS2 and JAW1 are expressed in all TGCTs, whereas one 12p amplicon-positive TGCT lacks expression of SOX5. The critical region of 12p over-represented in TGCTs is less than 8% of the total length of the short arm of chromosome 12. It will be helpful in the identification of the gene(s) involved in TGCT-development.

Mental status and fragile X expression in relation to FMR-1 gene mutation.

The fragile X mental retardation syndrome is caused by unstable expansion of a CGG repeat in the FMR-1 gene. Clinical expression is associated with a large expansion of the CGG repeat. The mutation in the FMR-1 gene and the cytogenetic expression of the fragile site at Xq27.3 have been studied in 52 fragile X male patients. The percentage of the cytogenetic expression of the fragile site at Xq27.3 positively correlates with the mean size of the full mutation in the FMR-1 gene (p < 0.0001) irrespective of the presence of additional premutation alleles. We noted a less frequent occurrence of additional premutation alleles in adult patients compared with juveniles, suggesting a continued mitotic instability in life. Additionally, the level of mental retardation has been ascertained in 35 patients using the Stanford-Binet or Terman-Merrill test of general intelligence. The presence of a full mutation in the FMR-1 gene seemed decisive for the occurrence of mental impairment in the patient. No correlation is observed between the degree of mental retardation and the size of the full mutation. The degree of mental retardation seemed not to be influenced by the presence of premutation alleles in part of the cells in addition to a full mutation. One patient is described with the 'Prader-Willi-like' subphenotype of the fragile X syndrome, showing a deletion in the FMR-1 gene in a part of his cells in addition to a full mutation.

Limited size of the fragile X site shown by fluorescence in situ hybridization.

Cosmids, isolated from a 475 kb YAC that spans the fragile X region, and the YAC itself, were used for fluorescence in situ hybridization (FISH) on metaphase chromosomes from fragile X patients. Cosmid 22.3, containing most of the hybrid translocation breakpoints, shows in situ hybridization signals distal and proximal from the fragile X site. We propose that the size of the fragile site is limited to 20 kb.

No apparent involvement of the FMR1 gene in five patients with phenotypic manifestations of the fragile X syndrome.

Most fragile X patients have a significant increase in the number of CGG repeats in the FMR1 gene. Two patients were described with a deletion and one patient with a point mutation in the FMR1 gene. We describe 5 patients with a fragile X or Martin-Bell phenotype. Two brothers were discordant for the region containing the FMR1 gene; if there is a common cause for the mental retardation this is not located in the FMR1 gene. In the other 3 patients the expression of the FMR1 gene was found to be normal and no abnormalities were noted in the FMR1 mRNA. No amplification was found in the GCC repeat which is associated with the fragile site FRAXE. We conclude that the Martin-Bell phenotype can also be caused by mutations outside the FMR1 gene.

Intragenic probe used for diagnostics in fragile X families.

The intragenic (FMR-1) probe pE5.1 was used for DNA analysis in fragile X families. With this probe fragments of altered size can be detected in female carriers, affected individuals and transmitting males. The length of the altered fragments was found to vary from one generation to another as well as between sibs. This instability of the DNA detected by pE5.1 was also seen in peripheral blood within single individuals. These phenomena are illustrated by 4 exemplary families segregating the fragile X syndrome. We demonstrate the diagnostic contribution of intragenic analysis to carrier detection as well as the identification of normal transmitting males carrying premutations. One of the families illustrates the passage of a premutation to a male through 2 generations.

Genomic imprinting in testicular germ cell tumours.

Genomic imprinting refers to the parental origin-specific functional difference between the paternally and maternally-derived mammalian haploid genome. Normal embryogenesis depends on the presence of both a paternal and a maternal copy of particular chromosomal regions, containing the so-called imprinted genes. Genomic imprinting is established somewhere in the maturation from a primordial germ cell to a mature gamete, either spermatid or oocyte. We discuss the value of testicular cancers, especially those derived from the germ cell lineage, as a model to study erasement of the biparental pattern of genomic imprinting as present in the zygote and establishment of the paternal pattern during spermatogenesis. In addition, we will present data on the presence of X-inactivation in these cancers.

Distinctive Phenotypic Abnormalities Associated with Submicroscopic 21q22 Deletion Including DYRK1A.

Partial monosomy 21 has been reported, but the phenotypes described are variable with location and size of the deletion. We present 2 patients with a partially overlapping microdeletion of 21q22 and a striking phenotypic resemblance. They both presented with severe psychomotor delay, behavioral problems, no speech, microcephaly, feeding problems with frequent regurgitation, idiopathic thrombocytopenia, obesity, deep set eyes, down turned corners of the mouth, dysplastic ears, and small chin. Brain MRI showed cerebral atrophy mostly evident in frontal and temporal lobes, widened ventricles and thin corpus callosum in both cases, and in one patient evidence of a migration disorder. The first patient also presented with epilepsy and a ventricular septum defect. The second patient had a unilateral Peters anomaly. Microarray analysis showed a partially overlapping microdeletion spanning about 2.5 Mb in the 21q22.1-q22.2 region including the DYRK1A gene and excluding RUNX1. These patients present with a recognizable phenotype specific for this 21q22.1-q22.2 locus. We searched the literature for patients with overlapping deletions including the DYRK1A gene, in order to define other genes responsible for this presentation.

Genetic and clinical analysis of a large Dutch Gilles de la Tourette family.

Gilles de la Tourette syndrome is a complex neuropsychiatric disorder, which becomes evident in childhood between the ages of 2 and 15 years. Tourette syndrome is defined by the occurrence of a large range and variable number of unwanted repetitive simple or complex motor and vocal tics that start in childhood and follow a waxing and waning course. A major gene for this syndrome has not yet been identified, probably owing to both genetic and phenotypic heterogeneity of this disease. This article describes the clinical evaluation of patients and family members in a large Dutch Gilles de la Tourette Syndrome pedigree and the decisions encountered with respect to phenotyping. The importance of an accurate definition of the Tourette phenotype is discussed, which is highly important for reliable genetic linkage and association studies. Subsequent linkage analysis resulted in three linkage peaks on different chromosomes 3q, 9q, and 13q. Multipoint analysis resulted in a single linkage peak with logarithm of odds score 2.55 with marker D3S1311 on chromosome 3q.

The fragile X syndrome: isolation of the FMR-1 gene and characterization of the fragile X mutation.

Fragile X syndrome, associated with the fragile X chromosome, is the most common cause of familial mental retardation. A breakthrough has been made in molecular biological research into the fragile X site. In this review we describe the molecular investigations that have led to the isolation of the FMR-1 gene. The nature of the fragile X mutation as well as the implications of the DNA test for the mutation are discussed.

X inactivation in human testicular tumors. XIST expression and androgen receptor methylation status.

In female mammalian cells, inactivation of one of the X chromosomes compensates the increased dosage of X-linked genes as compared with their male counterparts. This process is initiated by the X-inactive specific transcripts of the xist/XIST gene in cis, resulting in methylation of specific sites of genes to be silenced. However, in male germ cells, X inactivation is established by xist/XIST expression only. We investigated the X inactivation pattern in human testicular tumors of different histogenesis by analysis of XIST expression and methylation of the androgen receptor gene. XIST was expressed only in tumors derived from the germ cell lineage with supernumerical X chromosomes: seminomas, nonseminomas, and spermatocytic seminomas. Although low expression was present in testicular parenchyma with spermatogenesis, XIST was expressed at a higher level in parenchyma with carcinoma in situ, the precursor lesion of seminomas and nonseminomas. Despite the consistent expression of XIST in germ-cell-derived tumors with gain of X chromosomes, methylation of the androgen receptor gene was present in all differentiated but only in a proportion of the undifferentiated nonseminomas. This differential pattern of methylation was also found in a number of representative cell lines. Our data indicate that the counting mechanism resulting in X inactivation is functional in testicular cancers of different histogenesis. Moreover, the differentiation-dependent pattern of X inactivation as reported during normal development in the case of multiple X chromosomes by methylation is retained in these tumors. We conclude therefore that X inactivation allows the excessive gain of X chromosomes found in germ-cell-derived tumors of the adult testis. In addition, this offers an interesting model to study the fundamental mechanisms of these processes.

Heterogeneous X inactivation in trophoblastic cells of human full-term female placentas.

In female mammalian cells, one of the two X chromosomes is inactivated to compensate for gene-dose effects, which would be otherwise doubled compared with that in male cells. In somatic lineages in mice, the inactive X chromosome can be of either paternal or maternal origin, whereas the paternal X chromosome is specifically inactivated in placental tissue. In human somatic cells, X inactivation is mainly random, but both random and preferential paternal X inactivation have been reported in placental tissue. To shed more light on this issue, we used PCR to study the methylation status of the polymorphic androgen-receptor gene in full-term human female placentas. The sites investigated are specifically methylated on the inactive X chromosome. No methylation was found in microdissected stromal tissue, whether from placenta or umbilical cord. Of nine placentas for which two closely apposed samples were studied, X inactivation was preferentially maternal in three, was preferentially paternal in one, and was heterogeneous in the remaining five. Detailed investigation of two additional placentas demonstrated regions with balanced (1:1 ratio) preferentially maternal and preferentially paternal X inactivation. No differences in ratio were observed in samples microdissected to separate trophoblast and stromal tissues. We conclude that methylation of the androgen receptor in human full-term placenta is specific for trophoblastic cells and that the X chromosome can be of either paternal or maternal origin.

Genetic mapping on the mouse X chromosome of human cDNA clones for the fragile X and Hunter syndromes.

Murine X-linked genes corresponding to the human Fragile X (FMR1) and Hunter syndrome (IDS) loci have been mapped in an interspecific backcross between B6CBA-Aw-J/A-Bpa and Mus spretus using human cDNA clones. Pedigree analysis of recombinants from a total of 248 backcross progeny favors a gene order of (Cf-9, Mcf-2)-(Fmr-1)-Ids-Gabra3-Rsvp. Gene order is conserved between the species, although no fragile site has been detected in the mouse in this region of the murine X chromosome.

Methylation similarities of two CpG sites within exon 5 of human H19 between normal tissues and testicular germ cell tumours of adolescents and adults, without correlation with allelic and total level of expression.

Testicular germ cell tumours (TGCTs) of adolescents and adults morphologically mimic different stages of embryogenesis. Established cell lines of these cancers are used as informative models to study early development. We found that, in contrast to normal development, TGCTs show a consistent biallelic expression of imprinted genes, including H19, irrespective of histology. Methylation of particular cytosine residues of H19 correlates with inhibition of expression, which has not been studied in TGCTs thus far. We investigated the methylation status of two CpG sites within the 3' region of H19 (exon 5: positions 3321 and 3324) both in normal tissues as well as in TGCTs. To obtain quantitative data of these specific sites, the ligation-mediated polymerase chain reaction technique, instead of Southern blot analysis, was applied. The results were compared with the allelic status and the total level of expression of this gene. Additionally, the undifferentiated cells and differentiated derivatives of the TGCT-derived cell line NT2-D1 were analysed. While peripheral blood showed no H19 expression and complete methylation, a heterogeneous but consistent pattern of methylation and level of expression was found in the other normal tissues, without a correlation between the two. The separate histological entities of TGCTs resembled the pattern of their nonmalignant tissues. While the CpG sites remained completely methylated in NT2-D1, H19 expression was induced upon differentiation. These data indicate that methylation of the CpG sites within exon 5 of H19 is tissue dependent, without regulating allelic status and/or total level of expression. Of special note is the finding that, also regarding methylation of these particular sites of H19, TGCTs mimic their non-malignant counterparts, in spite of their consistent biallelic expression.

Alternative splicing in the fragile X gene FMR1.

The FMR1 gene, associated with fragile X syndrome, has recently been cloned and the sequence of partial cDNA clones is known. We have determined additional cDNA sequences both at the 5' and 3' end. We have characterized the expressed gene by means of RT-PCR in various tissues and have found that alternative splicing takes place in the FMR1 gene, which does not seem to be tissue specific. When the different alternative splicing events are combined, 12 distinct mRNA products could result from FMR1 expression in each tested tissue. In all these transcripts the open reading frame is maintained until the same stop codon. At the 3' end alternative use of polyadenylation signals is found. The alternative splicing allows functional diversity of the FMR-1 gene. Whether all the possible proteins will be synthesized and whether they will be functionally active has to be determined.

Variation of the CGG repeat at the fragile X site results in genetic instability: resolution of the Sherman paradox.

Fragile X syndrome results from mutations in a (CGG)n repeat found in the coding sequence of the FMR-1 gene. Analysis of length variation in this region in normal individuals shows a range of allele sizes varying from a low of 6 to a high of 54 repeats. Premutations showing no phenotypic effect in fragile X families range in size from 52 to over 200 repeats. All alleles with greater than 52 repeats, including those identified in a normal family, are meiotically unstable with a mutation frequency of one, while 75 meioses of alleles of 46 repeats and below have shown no mutation. Premutation alleles are also mitotically unstable as mosaicism is observed. The risk of expansion during oogenesis to the full mutation associated with mental retardation increases with the number of repeats, and this variation in risk accounts for the Sherman paradox.